1. Field of the Invention
The present invention relates to a treating agent for forming a fluoride coating film and a method for forming a fluoride coating film.
2. Description of Related Art
NdFeB rare earth sintered magnets are excellent in magnetic properties, and thus used as large magnets represented by motors for automobiles and high-performance magnets such as thin magnets represented by spindle motors. The NdFeB rare earth magnets are expected for wider applications in the future. However, there are requests to further improve the performance of the magnets. As for the magnets represented by motors for automobiles which are used in high temperature atmospheres and are required to be reliable, demanded are not only improved heat resistance, namely no degradation of magnetic properties in high temperatures, but also an increased coercive force. On the other hand, the thin magnets have a problem of degradation of magnetic properties caused by damaged surface layers generated during processing magnets into thin magnets. The thinner a magnet is, the worse the degradation of magnetic properties is (WO2006/043348).
An approach to deal with the problem is under review which intends to improve magnetic properties of a damaged layer and magnetic properties of a magnet by subjecting a magnetic article to a surface treatment.
On the other hand, in order to improve magnetic properties of rare earth magnets, an approach is under review in which material magnetic particles, which is a preform of a magnetic article, is subjected to a surface treatment using a fluorine compound (JP 2006-66870 A).
In order to improve magnetic properties of NdFeB rare earth sintered magnets, it is stated that a surface treatment is effective using a Dy compound and a Tb compound, in particular a Dy fluoride and a Tb fluoride (WO2006/043348). According to the conventional techniques described in WO2006/043348, a fluorine compound constitutes a granular grain boundary phase and is not arranged along the grain boundary or the powder surface of a magnet. In order to diffuse a surface treating agent component into a magnet for the purpose of improving magnetic properties of the magnet with the surface treating agent, a magnet component is required to be partially liquefied because the magnet comes into contact with the surface treating agent in points. Furthermore, because the Dy compound and the Tb compound are thermally stable, a magnet subjected to the surface treatment is required to be subjected to a heat treatment (absorption treatment) at 800° C. or higher for 1 hour or more to improve magnetic properties of the magnet. Therefore, in order to improve magnetic properties of NdFeB rare earth sintered magnets, an ageing treatment is required, and thus two heat treatments are required. The distance of the diffusion into magnets of a Dy compound and a Tb compound in the absorption treatment is limited to several millimeters. Therefore, it is difficult to improve magnetic properties but for thin magnets.
On the other hand, it is considered effective to omit the absorption treatment, or to apply a Dy compound and a Tb compound to magnetic particles for improving magnetic properties of not thin, large magnets. However, when the difference between the average particle diameter of magnetic particles and the average particle diameter of a Dy fluoride and a Tb fluoride are within two digits, additional amounts of the Dy fluoride and the Tb fluoride become nonnegligible in relation to the volume fraction of the Dy fluoride and the Tb fluoride based on the magnetic particles. A Dy fluoride and a Tb fluoride are non magnetic. Therefore, addition of large amounts of a Dy fluoride and a Tb fluoride in subjecting magnetic particles to a surface treatment increases the coercive force of the treated magnet but invites decrease of magnetic flux density, thus not resulting in improvement of magnetic properties.
The present inventors have investigated and found that for the purpose of improving magnetic properties of NdFeB rare earth magnets including not only sintered magnets but also bonded magnets, it is effective to stack layers containing a rare earth fluoride or an alkaline earth metal fluoride, in particular, a fluoride of Pr, Nd, Dy, Tb or Ho with proper layer thicknesses on the surfaces of material magnetic particles or the surface of a thin magnet.
An object of the present invention is to provide a treating agent for forming a fluoride coating film containing a rare earth fluoride or an alkaline earth metal fluoride, in particular, a fluoride of Pr, Nd, Dy, Tb or Ho, that improves magnetic properties of NdFeB rare earth magnets including not only sintered magnets but also bonded magnets; and a method for manufacturing the treating agent for forming a fluoride coating film.